pressure dependence of dissociation fraction and optical emission characteristics in low pressure inductively coupled n2 ar plasmas

Pressure dependence of dissociation fraction and optical emission characteristics in low-pressure inductively Department of Physics, Dong-A University, Busan 604-714, Korea Received 8 Ap

low-pressure inductively coupled N2-Ar plasmas

T H Chung, Y W Lee, H M Joh, and M A Song

Citation: AIP Advances 1, 032136 (2011); doi: 10.1063/1.3628670

View online: http://dx.doi.org/10.1063/1.3628670

View Table of Contents: http://aip.scitation.org/toc/adv/1/3

Published by the American Institute of Physics

Pressure dependence of dissociation fraction and optical emission characteristics in low-pressure inductively

Department of Physics, Dong-A University, Busan 604-714, Korea

(Received 8 April 2011; accepted 20 July 2011; published online 12 August 2011)

per-formed by using optical emission spectroscopy (OES) and an rf-compensated Langmuir probe under the conditions of pressures of 1 30 mTorr and powers of 300

-600 W In the OES experiments, the argon was used as an actinometer and as an adding gas The effect of the argon content in the gas mixture was examined in the range of 5 - 30% The investigation of the effects of pressure on the dissociation frac-tion of nitrogen molecules and on the optical emission characteristics were carried out The correction factors for estimating the dissociation fraction by OES actinom-etry accounting for argon effect were formulated and calculated It was found that the dissociation fraction increased with increasing power and Ar content, while it de-creased with increasing pressure In addition, the electron energy probability function (EEPF), the electron density, and the electron temperature were obtained by using

a Langmuir probe to investigate the effects of the plasma parameters on the optical

emission characteristics and the dissociation fraction Copyright 2011 Author(s) This

article is distributed under a Creative Commons Attribution 3.0 Unported License.

I INTRODUCTION

in the synthesis of nitride thin films and in the surface modification of various materials Since atomic nitrogen plays an important role in the plasma processes, the determination of the absolute

growing interest in the application of inductively coupled plasma (ICP) sources for numerous plasma-enhanced materials processing It has been known that most of these plasma systems are

The dissociation fraction in an inductively coupled nitrogen plasma is important for understand-ing and improvunderstand-ing the nitridation processes because the number density of N atoms is deducible from the dissociation fraction Generally, it is difficult to achieve a high dissociation efficiency of

Czerwiec et al measured the dissociation fraction for an ICP sustained in a long cylindrical tube with

a small radius specially designed for radical beam generation They obtained a dissociation fraction from 0.1 (evaluated by using optical emission actinometry) up to 0.7 (by using mass spectrometry)

dissociation of molecular nitrogen is to introduce another gas such as hydrogen and argon in the

plasma and observed the dissociation enhancement factor of 4.1 - 8.5 with the introduction of Ar to

charges species and neutrals (atomic and molecular nitrogen and argon atoms) including the wall

sig-nificant changes in the properties of discharge such as gas temperature, rovibrational excitation,

caused an enhancement of the dissociation fraction It was found out that the calculated density of

with larger portion of Ar would give an overestimated value for dissociation fraction because the Penning excitation and/or dissociative excitation of nitrogen molecules due to all the excited states

of Ar is usually neglected In this work, we will determine the dissociation fraction more accurately

by estimating the contributions from Ar excited states in more detail

Another emphasis is placed on the effect of pressure on the dissociation fraction and discharge characteristics For that purpose, the Ar content in the gas mixture is kept constant at 5% otherwise mentioned This amount of Ar content allows us to apply the optical emission actinometry But, as the pressure increases the amount of Ar gets larger, thus the interactions of the Ar atoms with neutrals and ions of atomic and molecular nitrogen become important Especially, argon metastable atoms play an important role in the discharge kinetics, thereby influencing the dissociation of nitrogen molecules as well as rovibrational temperature In this study, the density of argon metastables is calculated using a simple kinetics This quantity is used to make a correction for determining the dissociation fraction by the optical emission actinometry

The total pressure is varied in an attempt to fully characterize the optical emission characteristics

emission actinometry at various powers (300 - 600 W) in the pressure range 1 - 30 mTorr, which

dissociation fraction with pressure is given with a discussion of the particle balance and of plasma properties, such as the electron density, the electron temperature, and the electron energy probability function measured by using a rf-compensated Langmuir probe

II EXPERIMENT

A schematic diagram of the experimental setup with the diagnostics system (optical emission

consists of a stainless-steel cylinder with a 28-cm diameter and a 34-cm length A 1.9-cm-thick by 27-cm-diameter tempered glass plate mounted on one end separates the planar one-turn induction coil from the plasma The induction coil is made of copper (with water-cooling) and is connected to

an L-type capacitive matching network and a rf power generator

The plasma chamber is evacuated by using a diffusion pump backed by rotary pump giving a

combination vacuum gauge (IMG 300) The operating gas pressure is controlled by adjusting the mass flow controller The nitrogen gas pressure is varied in the range of 1 - 30 mTorr, and a 13.56 MHz generator (ENI OEM 12) drives an rf current in a flat one-turn coil through the rf power generator

gas for all cases

FIG 1 (Color online) Schematic diagram of experimental set-up and diagnostics system.

An rf-compensated cylindrical single Langmuir probe was mounted through one of the ports

on the vacuum chamber The probe tip was located on the axis of the cylinder at 14 cm below the tempered glass plate To measure the plasma parameters, the harmonic technique, which exploits the generation of harmonics resulting from excitation of the nonlinearity of the single Langmuir probe characteristics, combined with Druyvesteyn method was used In the harmonic method, the voltage

of a sawtooth wave After being amplified by operational amplifiers, the sinusoidal signal was

amplified by a power amplifier, and then applied to the probe tip though a resonance filter for 13.56 and 27.12 MHz to remove the rf fluctuation from the plasma potential A cylindrical probe tip made

of tungsten which is 0.1 mm in diameter and 10 mm in length was used The current was obtained by

After data processing in the analog-to-digital converter, the fast Fourier transform was performed to

f () = 2m

e2S



2eV

m

1/2 d2I

where e is the electron charge, S is the probe area, m is the mass of electron, V is the probe potential

n e=

  max

0

f ( )d, T e= 2

3n e

  max

0

 f ()d, (2)

can also be determined from the slope of the probe I-V curve in the exponential region (from the point

where the probe current is zero to where the slope of the curve begins to decrease) We observed that both methods yield almost same values of the electron temperature The EEDF integral method has

The light intensity of emissive molecules and radicals in the plasma was focused by means

of optical fiber into entrance slit of 0.75 m monochromator (SPEX 1702), equipped with a grating

where a photomultiplier tube (Hamamatsu R928) converted photons into an electric signal Optical emission spectra were recorded in the wavelength range of 250 - 850 nm with a resolution of 0.1 nm The measured emission spectra should be corrected for the spectral response of the detection system which includes optical fiber, monochromator, and photomultiplier tube The detection system had to

be calibrated in intensity between 250 to 850 nm using a quartz halogen lamp with a known spectral radiance.The dependence of the emission intensities on the plasma parameters is investigated In plasma processing, actinometry is a frequently-used and well-developed technique to estimate the density of neutrals In this method, a known concentration of an impurity is introduced, and the intensities of two neighboring spectral lines, one from the known gas and one from the sample, are compared Since both species are bombarded by the same electron distribution and the concentration

of the actinometer is known, the density of the sample can be calculated

A kinetic analysis of a nitrogen discharge under the assumption of quasi-static equilibrium gives

dir

N + n e[Nm]k exc N m + n e[N2]k di ss N2 −exc+ [Arm][N]k N Penn+ [Ar∗][N

2]k N Penn −diss

1

τ N + k Q

(3)

through resonant energy transfer called Penning excitation and dissociation

The emission intensity due to a transition from an excited level to a lower state is

I (N∗ = K N hν N A N[N∗ , (4)

I (N∗ = K N hν N A N τ N n e [N]k dir N (1+ c1). (5)

have

c1= n e[N

m]k exc

N m + n e[N2]k di ss N2 −exc+ [Arm][N]k Penn

The rate coefficients k exc N m , k di ss −exc

N2 , k Penn

N , k Penn −diss

Similarly, the emission intensity from excited Ar atom is written as

I (Ar∗ = K Ar h ν Ar A Ar τ Ar n e [Ar]k dir(1+ c2), (7)

0 2 4 6 8 10

10 -9

10-8

10-7

10-6

K exc Ar m

K Ar

m

e

K i e

e

Te (eV)

3 s -1 )

c2=[Arm]k

exc

Ar m

Density of metastable atoms is determined from the balance of the production (mainly by

equation under the assumption of a quasi-static equilibrium gives

m

e n e[Ar]

(k exc

e + k Q

e + k i

e )n e + k Q

N2[N2]+ k di f f

e , k i

e , and k Q

The rate coefficients are calculated using the cross section data as

k e j =



2e

m

  max

0

σ j()√ f ()d, (10)

1s5(k e Ar m), the direct excitation of Ar metastables to 2p1state (k exc Ar m), and the total direct excitation

because they are not sensitive to two step excitation

[N]

I750

(1+ c2) (1+ c1)

K750

K746

ν750

ν746

A750

A746

k dir Ar

k dir

τ750

τ746. (11)

Then the dissociation fraction is derived as by9,11

[N]

I750

(1+ c2) (1+ c1)

k dir Ar

k dir N

x Ar

x N2

off However, quantitatively accurate results can only be obtained if excitations via dissociative channels, Penning effect, and the quenching of excited states are accounted for

In this experiment, using OES, we obtain the dissociation fraction for an inductively coupled

understand the effects of these parameters on the dissociation fraction, we measured EEPF (electron energy probability function), and the electron density, and the electron temperature by using a Langmuir probe

III RESULTS AND DISCUSSION

1.4 mTorr at ICP power 500 W The main emission peaks correspond to several transition lines

g (v)→ A3+

u (v)→ B3

u (v)→ X2+

g (v))

u,

of all peaks significantly increased with power, while they overall decreased with pressure The

and quenching processes such as the electron impact excitation from the ground state, the excitation

in a slight increase of the emission intensity from N atoms

plasmas at different pressures Many SPS bands (v = −2, −1, 0, +1, +2, +3, +4) and FNS bands (v = 0, +1) are clearly observed Although not shown in the figure, the intensities from SPS at

5 mTorr increased compared to those at 1.4 mTorr But, as the pressure further increases, the emission intensities of these bands decrease The relative intensity of each bands vary depending on

thus the electron-impact vibrational excitation to higher levels are reduced However, the number

200 300 400 500 600 700 800 900 0

2 4 6 8

Wavelength (nm)

Ar 5% 1.4 mTorr 600 W

Ar N(746.8 nm)

(a)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

0.0

0.5

1.0

1.5

2.0

2.5

0

2

4

6

8

10

12

0 1 2 3 4 5 6 7 8

400 410 420 430 440 450

(1,3)

(1,0)

(0,1) (0,0)

(0,0)

(d)

Wavelength (nm)

30 mTorr, Ar 5%, 600 W

(2,5)

Δ ν = + 3

Δ ν = + 2

Δ ν = + 1

Δ ν = 0

Δ ν = - 1

(0,2) (1,3)

(0,0)

(c)

(1,4)

(2,6) (4,8)

Wavelength (nm)

22 mTorr, Ar 5%, 600 W

(0,3)

Δ ν = - 2

(1,4) (3,1)

(2,0)

(4,4)

(2,5) (0,0) (3,5) (2,4)

(1,2) (0,1)

(3,2)

(2,1)

Wavelength (nm)

600 W

(1,0)

(0,0)

(a)

(1,3) (0,1) (1,0)

(0,0)

(0,0)

(b)

Wavelength(nm)

11 mTorr, Ar 5%, 600 W

Ar

Ar

(4,8)

(2,6) (3,7)

(1,5)

(2,3) (0,1) (1,2)

Ar Ar Ar (1,5)

(0,4) (3,7) (2,6)

(4,8) (0,3)

(0,1) (1,2) (2,3)

Ar

Ar Ar

(1,5) (2,6) (3,7) (0,3)

represent the emission lines between 400 nm and 450 nm.

also deexcitation) increases again at 22 mTorr In the insets, the spectra in the wavelength range

the FNS bands (0,1) at 427.8 nm, (1,2) at 423.6 nm, and (2,3) at 419.9 nm are observed The Ar

700 720 740 760 780 800 820 840 0.0

0.2 0.4 0.6

0.0

0.2

0.4

0.6

0.0

0.2

0.4

0.6

0.0 0.2 0.4 0.6

Wavelength (nm)

30 mTorr, Ar 5%, 600 W

N N

746.8

763.5 750.4

811.5

Wavelength (nm)

22 mTorr, Ar 5%, 600 W

N

738.3

810.4

N2(3,1)

N2(4,2) N

2 (5,3)

N

811.5

Wavelength (nm)

600 W

N

696.5

750.4 763.5

746.8

N

801.4 794.8 772.4 714.7

N2(2,0)

746.8

750.4 696.5

Wavelength (nm)

11 mTorr, Ar 5%, 600 W

811.5

intensi-ties of these lines decrease The optical spectrum from the 11 mTorr discharge shows a significant change compared to that of the 5 mTorr discharge The argon peaks except some dominant ones are

pressure However, when the gas pressure increased to 22 mTorr, with an abundant supply of nitrogen molecules, the emission from nitrogen atom is clearly seen At the pressure of 30 mTorr, the N peaks diminish again while the Ar 811.5 nm line becomes high

Langmuir probe measurement as a function of the Ar content With an increase in the Ar content, the electron density increases and the electron temperature decreases For a fixed power, with an increase of Ar content, the total energy loss per electron-ion pair decreases, hence the electron density increases due to the power balance This trend is in agreement with the modeling and the

the Ar content of 10 - 30 % at the ICP power of 500 W and the pressure of 1.4 mTorr In this work, the electron energy probability functions were found to be Maxwellian The population of electrons with energy greater than 15 eV exhibits an unstable behavior, which might be caused by noise For nitrogen, the threshold for dissociation is at 9.8 eV; however, the cross section function does not rise

0 5 10 15 20

107

108

109

10 15 20 25 30 8x10 9

10 10

1.2x10 10

1.4x10 10

10 15 20 25 30 3.0

3.2 3.4 3.6

(c)

Energy (eV)

Ar 10%

Ar 15%

Ar 20%

Ar 30%

-3 )

1.4 mTorr 500 W

-3 )

Ar content (%)

1.4 mTorr 500 W

Ar content (%)

1.4 mTorr 500 W

FIG 6 (Color online) (a) Electron density and (b) electron temperature obtained by using a Langmuir probe (c) Electron

above 20 eV However, ICP discharges show a considerable amount of high energy electron,

in a higher dissociation fraction This is also related to an increase in the relative production of Ar metastables

increases first and has a maximum at 11 mTorr, and then slightly decreases with increasing pres-sure, which is due to the increased collision frequency between electrons and neutral molecules or atoms The electron temperature decreases with increasing pressure: for the ICP power of 500 W,

EEPF with different gas pressures at the ICP power of 500 W The population of electrons with high energy exhibits an unstable fluctuation As pressure increases, an easy heating due to a fre-quent electron-impact rovibrational excitations of nitrogen molecules is thought to contribute to the fluctuation of the probe currents However, the shape of EEPF roughly remains the Maxwellian distribution At the pressure of 22 mTorr, there is a significant decrease of the electron energy

in the high energy electron region, while the electron density in the low energy electron region does not change much compared to that of 11 mTorr This can be explained by that the depletion

of the high energy electrons around the excitation threshold becomes dominant at the pressure of

22 mTorr